Many relatively large turbine engines, including turbofan jet engines, may use an air turbine starter (ATS) to initiate turbine engine rotation. The ATS may be mounted by the engine, much as a starter for an automobile is located by the automobile engine. The ATS may be coupled to a high pressure fluid source, such as compressed air, which impinges upon a turbine wheel in the ATS causing it to rotate at a relatively high rate of speed. The ATS includes an output shaft that is coupled to the turbine wheel and, perhaps via one or more gears, to the jet engine. The output shaft thus rotates with the turbine wheel. This rotation in turn causes the jet engine to begin rotating. The applicant for the present invention, Honeywell International, Inc., has for years successfully designed, developed, and manufactured ATSs.
The flow of compressed air to an ATS may be controlled by, for example, a valve. This valve, if included, is typically referred to as a starter air valve. When the starter air valve is open, compressed air may flow through the starter air valve, and into the ATS. Conversely, when the starter valve is closed, compressed air flow to the ATS may be prevented. An ATS starter air valve, in many instances, includes a pneumatic actuator to move the valve into its open position. The source of pneumatic power to the actuator may be pressurized air supplied from, for example, an auxiliary power unit (APU), bleed air from another engine compressor, or a ground cart. In some instances, the pressurized air supplied to the ATS and the starter air valve is non-regulated, and at a pressure magnitude greater than what may be needed for the ATS operation. Hence, some aircraft starter air valves may also be configured as a pressure regulating valve, to thereby regulate the pressure of the air flow to the ATS.
Many starter air valve pneumatic actuators, for both regulator and non-regulator types of valves, include pistons with either a dynamic seal or diaphragm. Many of these actuators also include a small, fixed-diameter rating orifice to help control the opening rate of the starter air valve, and the pressure rise rate downstream of the starter air valve. Although these present actuators generally operate safely and effectively, the actuators can suffer certain drawbacks. For example, the piston seal (or diaphragm) can wear and cause some leakage past the actuator piston. This leakage can potentially degrade the opening performance of the starter air valve. In some cases, if the leakage becomes great enough, the actuator may be unable to open the starter air valve.
Hence, there is a need for a pneumatic control for various valves, including starter air valves, that addresses the above-noted drawbacks. Namely, a pneumatic control that will compensate for piston seal or diaphragm leakage that may occur in the actuator and/or will continue to operate even for relatively large amounts of piston seal or diaphragm leakage. The present invention addresses one or more of these needs.